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jmateus 8th November 2009 12:39 PM

Spoeaker protection, project #33
Hi folks

I need some help on project #33, Rod Elliot, louspeaker protection.

I built the circuit and it seems to be working fine, except for one "small" detail, when
I first turn on the amplifier the relays engage right away, there is no delay whatsoever, even though Rod states that there is a delay of at least 4 seconds.

I read and reread the article and I can't find any help from this reading. There is however the mentioning of increasing C1 to increase the delay time but that didn't work for me.

There must be some problem with the circuit. I believe the combination C1/R1
affects the bias of Q1 and probably the delay time, but I really can't pinpoint
the reason of this discrepancy.
I tried to e/mail Rod but there is no provision for that in his Web site.

Is there anyone that built the circuit and could help me out?
I'd appreciate any suggestions.

BobEllis 8th November 2009 01:50 PM

Check that the relay controller section (Figure 2) does not turn on with the "Off" terminal shorted to ground. If it does the problem is in that part of the circuit.

Otherwise look to the mute circuit, figure 3, which is what gives you the turn on delay. If R3 is too low that would make the circuit turn on too fast. Is C1 the proper value?

jmateus 9th November 2009 12:01 PM

Protection #33
Thanks Bob Ellis for your reply.
Yes, C1 is 1Uf, howeveer it is a polyester cap, does it a make a difference not being
an electrolytic? Shoul I increase the value, say, to 10uF?
Taking the OFF to ground doesn't do much except a spark that bothered me.
I thought Fig 2, relay activation circuit, was what determines the delay function, C1
and R1 being the constant time factor for the relay to operate.
I must be wrong thoug.

BobEllis 9th November 2009 01:51 PM

Oops - had it backwards - taking the off terminal high ought to deactivate the relays.

Looking at it again, C1/R1 in fig 2 ought to make a delay. Perhaps R1 or C1 is too small?

jmateus 10th November 2009 12:46 PM

That's what I thought, C1/R1 in fig 2 make the time delay and that's what isn't working. I already increased C1 to 100uF with no avail.
Perhaps I would play with R1, wiil see. In any event I have a feeling something is wrong with this circuit. I'll fiddle with it, thanks for your interest, Bob.

BobEllis 10th November 2009 10:36 PM

you have a jumper or some R7 installed, right?

jmateus 11th November 2009 11:41 AM

Hi Bob

Yes, R7 is installed, no appretiable difference in the overall circuit.
In fact I tested the circuit for DC ands lack of AC and it works with no problem.
Except for that detail (no delay at turn ON) the circuit works flawlessly.

It's a cruel world this one, ah,ah.

MJL21193 11th November 2009 01:52 PM


Originally Posted by jmateus (
Thanks Bob Ellis for your reply.
Yes, C1 is 1Uf, howeveer it is a polyester cap, does it a make a difference not being
an electrolytic? Shoul I increase the value, say, to 10uF?

From the ESP page, C1 is shown to be 47uF not 1uF.

jmateus 12th November 2009 12:59 PM

Speaker protection #33
Yes, C1 is marked 47uF on fig. 2 - Relay activation circuit but the value of C1 on Fig 3 - Loss of AC detector is 1 uF, shown as an electrolytic. I used a polyester one.
But wich one are you guys refering to?

Thanks for the answers anyway

MJL21193 12th November 2009 02:11 PM

The C1 in fig.2 is part of the RC delay - that is the issue, right, not enough delay?
Have you followed Rod's directions with regard to current draw and impedance of the relays used? What is your supply rail voltage?

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